Connecting system for a transition duct in a gas turbine

ABSTRACT

A system for connecting a low-pressure jet to a transition duct in a gas turbine, in which the said low-pressure jets are arranged in sectors of two or three stator blades and are filled on to the ring of the jets. The platforms of the jets are provided with a channel inside which the transition duct is positioned by means of an edge. Furthermore, centering between the transition duct and the ring of the jets is achieved directly using centering means at three or more points.

The present application is a U.S. National Phase (371 application) ofPCT/EP01/07413.

BACKGROUND OF THE INVENTION

The present invention relates to a system for connecting a low-pressurejet to a transition duct which form part of a gas turbine.

Furthermore the present invention relates to a low-pressure jet whichforms part of a gas turbine.

Gas turbines are known to comprise a compressor to which ambient air isfed and then pressurised.

The pressurised air passes through a series of combustion chambersterminating in a jet in each of which an injector supplies fuel whichmixes with the air to form an air-fuel mix for combustion.

The combustion gases are then sent towards the turbine which transformsthe enthalpy of the gases which have been combusted in the combustionchamber into mechanical energy available to a user.

Twin-shaft gas turbines have a gas generator and a power turbine withindependent shafts which rotate at different speeds.

The power rotor is made up of a shaft which at one end supports thelow-pressure turbine discs and the bearing flange at the other end.

The hot gases generated in the gas generator have to be converted topower available to a user by a low-pressure turbine.

The low-pressure jets accelerate and direct the hot gases towards therotor blades, transferring the useful power to the turbine shaft.

In order to keep the housing of a gas turbine as cool as possible, thestator jets over which the hot gases pass are not fixed directly to thestator housing but instead are arranged in sectors, each sector beingmade up of two or three jets, and, are insulated from the stator housingby means of dividers made of low heat conductivity material.

It should be noted here that the transition duct is the connectingcomponent between the high-pressure blades and the low-pressure jets.

In particular, the transition duct generally consists of two concentriccylinders whose middle section may have a different shape so as to allowthe desired flow of hot gases.

The hot gases have to flow in a way that minimises mechanical and heatlosses. The hot gases are at very high temperatures and, therefore, thewalls of the transition duct must be able to withstand these conditionsand maintain their structural integrity.

Furthermore, the sealing between the transition duct and thelow-pressure jets is an important variable for ensuring the performanceof the motor and for maintaining the integrity of the outer housing ofthe gas turbine.

In order to promote understanding, of the technical problems to besolved by the invention, reference should now be made to the followingstate of the prior art.

Transition ducts of existing turbines may be divided into twocategories: high-thickness transition ducts (which are divided intosegments) and low-thickness transition ducts (which are parts at 360°).

In both solutions, the housings and the low-pressure jets can easilysupport the external walls.

The main problem to be solved is that of correctly supporting theinternal wall which generally faces the rear of the high-pressure rotor.

Therefore, the internal wall may only be supported by an external wallusing structural struts or connecting arms or protuberances which extendfrom the platform of the low-pressure jets.

The first solution is particularly advantageous if the axial length ofthe transition duct is considerable, as the internal wall cannot beeasily supported at one end only.

However, the struts or connecting arms can cause mechanical losses andare not of simple design, precisely because of the high temperatures ofthe gases which flow through the transition ducts.

These high temperatures can cause incidents of “heat fatigue” which cancause various components to break and therefore significantly limittheir useful life and their resistance to heat fatigue.

In any case the axial length of the transition duct can only beminimised by extending the axial length of the platform of thelow-pressure jets so that the transition duct is included in theextended platforms of the low-pressure jets in some applications.

This solution is shown in FIG. 1 as an example of an embodiment of theprior art.

In particular FIG. 1 shows an assembly indicated generally by referencenumeral 20, comprising a transition duct 21 connected to the extendedplatform 22 of the low-pressure jets 23. A connecting arm 24 is alsoshown.

The segmented internal walls can also be supported by an internalstructure that is protected from hot gases, and are subject to weightloads and pressure differentials.

In the prior art, segmented transition ducts are commonly used forhigh-performance motors with horizontally separated housings alongsidethe (thin-walled) 360° transition ducts which are common in aeronauticalturbines.

BRIEF SUMMARY OF THE INVENTION

The object of the invention is therefore to provide a system forconnecting a low-pressure jet to a transition duct in a gas turbinewhich makes it possible to prevent heat transmission to the outerhousing through the centring system itself.

Another object of the invention is to provide a system for connecting alow-pressure jet to a transition duct in a gas turbine which avoids theuse of struts or connecting arms for its support and centring.

A further object of the invention is to provide a system for connectinga low-pressure jet to a transition duct in a gas turbine, which systemis simple and safe to install.

Yet a further object of the invention is to provide a system forconnecting a low-pressure jet to a transition duct in a gas turbinewhich is reliable, while bearing in mind the optimisation of theperformance of the machine, and which is economically viable.

These and other objects are achieved by a system for connecting alow-pressure jet to a transition duct in a gas turbine, in which thesaid low-pressure jets are arranged in sectors of two or three statorblades and are fitted on to the ring of the low pressure jets,characterised in that the platforms of the said low-pressure jets haveat least one channel inside which the transition duct is positioned bymeans of an edge and in that the centring between the said transitionduct and the said ring of the low-pressure jets is directly achieved bycentring devices operating at three or more points on the said ring ofthe low-pressure jets.

In a preferred embodiment of the invention, the centring means of thesaid transition duct consist of at least three keys which are forcedbetween the said low-pressure jets and the said transition duct. Thetransition duct has three slits, with each of the slits corresponding toone of three slits belonging to three sectors of the low-pressure jets,the whole having dimensions such as to hold the three keys in position.

More particularly, the three keys can be placed at three staggeredpoints at 120° each so as to ensure uniform support for the transitionduct.

In another preferred embodiment of this invention, the said low-pressurejet sectors directly include the abovementioned keys in their platform.

In yet another preferred embodiment of the invention, the channel of thering of the low-pressure jets is able to support the transition duct andgive it the correct axial position.

In yet another preferred embodiment of the invention, the transitionduct has the keys mounted or included in it and the low-pressure jetonly has a slit for the respective key.

In yet another preferred embodiment of the invention, at least threepoints on the edge of the transition duct have a predetermined gap withrespect to the channel for the low-pressure jet.

Furthermore, the predetermined gap is equal to the different thermalexpansions which the transition duct undergoes with regard to thelow-pressure jet, so that the gap is equal to zero when in a stationaryposition so as to allow the transition duct of the gas turbine to becentred with the channel while the edge forms the seal for the hot gasesof the turbine.

Finally, the invention also comprises a low-pressure jet which formspart of a gas turbine and having sectors with two or three statorblades, characterised in that its platform has at least one channelinside which a transition duct is positioned by means of an edge, and inwhich centring between the said transition duct and the said ring of thelow-pressure jets is directly achieved by centring means operating atthree or more points on the said ring of the low-pressure jets.

Further features of the invention are described in the appended claims.

The further objects and advantages of the invention and its structuraland functional features will become apparent from the description whichfollows and the appended drawings, which are provided by way ofillustration and are not intended to limit the scope of the invention,and in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a sectional view of a detail of a twin-shaft turbine,depicting a transition duct of the prior art;

FIG. 2 shows a sectional view of a detail of a twin-shaft turbine,depicting the system for connecting a low-pressure jet to a transitionduct according to the invention;

FIG. 3 shows a perspective view of the system for connecting alow-pressure jet to a transition duct according to the invention;

FIG. 4 shows a sectional view of the system for connecting alow-pressure jet to a transition duct according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

With particular reference to the figures mentioned, the system forconnecting a low-pressure jet to a transition duct in a gas turbineaccording to the invention is indicated generally by the referencenumeral 10.

In particular, the platforms 11 of the low-pressure jets 12 have achannel 13, inside which the transition duct 14 is positioned by meansof an edge 15.

The jets 12 are arranged in sectors of two or three stator blades andare fitted on to the ring of the low-pressure jets.

Therefore, these can be easily fitted onto the edge 15 of the transitionduct 14 so as to form the seal and the connection between thesecomponents.

Furthermore, the channel 13 of the low-pressure jets 12 supports thetransition duct 14 and also gives the transition duct 14 its correctaxial position.

A particularly important innovation is the system for centring the saidtransition duct 14 and the low-pressure jets 12.

As has been seen, in prior art gas turbines this centring was achievedby using the external housing of the gas turbine.

The improvement according to the present invention consists of thedirect centring of the transition duct 14 to the low-pressure jets 12.

This provides the significant advantage of avoiding heat transfer to theexternal housing through the centring system.

The material of the external housing is not suitable for hightemperatures and this new solution avoids oxidation, distortion or lossof sealing because of the high local temperature which would be createdin the external housing.

The system of the invention is assembled by using at least three keys 16among the low-temperature jets 12 and the transition duct 14.

Each key 16 has a cylindrical portion 30 with a central hole 32 and asquare portion 31.

The transition duct 14 has three slits 17 and the three correspondinglow-pressure jets sectors have another slit 18 or directly include thekeys 16 in their platform 11.

The resulting system is always centred, as the transition duct 14 runsalong the keys 16 and can only be coaxial with the ring of thelow-pressure jets 12 and with the stage of the jets 12, thus avoidingthe problems of the prior art referred to above.

More particularly, the three keys 16 are placed at three staggeredpoints at 120° each so as to ensure uniform support for the transitionduct 14.

Alternatively, the transition duct 14 may have the keys 16 fitted orincluded and the low-pressure jet 12 may only have the slit 18 for therespective key 16.

Another solution which has been envisaged consists of arranging at leastthree points on the edge 11 of the transition duct 14 which have apredetermined gap with the channel 13 of the low-pressure jet 12.

This predetermined gap is equal to the different thermal expansionswhich the transition duct 14 undergoes relative to the low-pressure jet12 so that the gap is equal to zero in a stationary position.

This phenomenon obviously allows the transition duct 14 to be centredwith the channel 13 whereas the edge 11 forms the seal for the hot gasesof the turbine.

Finally, the components of the system for connecting a low-pressure jetto a transition duct in a gas turbine, according to the inventiondescribed above, have the following important structural and functionalfeatures.

Firstly, the transition duct 14 is a part at 360° to the thin walls;secondly, the transition duct 14 does not have struts or connectingarms; finally, the transition duct 14 is supported by platforms 11connected to the low-pressure jets 12.

From the description given, the features of the inventive system forconnecting a low-pressure jet to a transition duct which forms part of agas turbine, which is the object of the invention, are obvious, as arethe advantages thereof.

We wish to state here the following conclusive considerations andobservations so as to specify the above advantages with greaterprecision and clarity.

A new solution to the problem referred to above has therefore beendeveloped to be implemented on a new turbine.

The axial length of the transition duct has been reduced and newtransition ducts and new connections to the low-pressure jets have beendeveloped.

It is obvious that numerous variants may be introduced to the inventivesystem for connecting a low-pressure jet to a transition duct in a gasturbine, the object of the invention, without departing from theinnovative principles contained in the idea illustrated.

It is obvious that this invention also comprises the relevantlow-pressure jet described above with its connection system to thetransition duct of the gas turbine.

It is finally obvious that in the practical use of this invention, thematerials, shapes and dimensions of the details illustrated may beanything, depending on requirements, and they may be replaced or variedby other items which are equivalent from a technical point of view.

The scope of the invention is defined by the appended claims.

1. A system for connecting a ring including a platform and sectors oftwo or more stator blades forming low-pressure jets to a transition ductin a gas turbine, said platform having at least one channel, saidtransition duct having an edge inside said channel and means forcentering the transition duct and the ring of the low-pressure jetsrelative to one another at three or more points on the ring of thelow-pressure jets.
 2. A connection system according to claim 1,characterised in that the centering means of the transition duct includeat least three keys forced between the low-pressure jets and thetransition duct.
 3. A connection system according to claim 1,characterised in that the transition duct has three slits where each ofthe slits corresponds to one of three slits forming part of threesectors of low-pressure jets so as to hold the three keys in position.4. A connection system according to claim 3, characterised in that thethree keys are placed at three staggered points of 120° each so as toensure uniform support for the transition duct.
 5. A connection systemaccording to claim 1, characterised in that the sectors of low-pressurejets directly include keys in said platform.
 6. A connection systemaccording to claim 1, characterised in that the channel of the ring ofthe low-pressure jets supports the transition duct in a predeterminedaxial position.
 7. A connection system according to claim 1,characterised in that said transition duct has keys and saidlow-pressure jet have slits for receiving the respective keys.
 8. Aconnection system according to claim 1, characterised in that at leastthree points of the edge of the transition duct have a predetermined gapwith the channel of the low-pressure jet.
 9. A connection systemaccording to claim 8, characterised in that the predetermined gap isequal to the different thermal expansions which the transition ductundergoes with respect to the low pressure jets, so that the gap isequal to zero in a stationary position, enabling the transition duct tobe centered with the channel while the edge forms the seal for the hotgases of the turbine.
 10. A connection system according to claim 1,characterised in that the transition duct extends about 360° and hasthin walls.
 11. A connection system according to claim 1, characterisedin that the transition duct is supported by platforms of thelow-pressure jets.
 12. A low-pressure jet forming part of a gas turbinewith sectors of two or three stator blades, characterised in that theplatform has at least one channel inside which a transition duct ispositioned by means of an edge and centering means between thetransition duct and the ring of the low-pressure jets is achieveddirectly by centering means operating at three or more points on thering of the low-pressure jets.